Signal peak limiter



Patented July 1, 1947 2,423,263 SIGNAL PEAK LIMITER Robert M. Sprague,Hicksville, N. Y., assignor to Press Wireless, Inc., Chicago, 111., acorporation of Delaware Original application December 30, 1941, SerialNo. 424,946. Divided and this application March 26, 1943, Serial No.480,618

This invention relates to electric signalling systems and moreespecially to control arrangements for use in telegraph systems,telefacsimile systems and the like.

Facsimile transmission can be roughly divided into two classes; onewhere the subject matter comprises a gradation of shades or half tonesbetween black and white; the other where the subject matter consistsonly of two shades, e. g., black and white. Ordinary printed or writtenmatter, line drawings and the like, would fall within the second class.In one known system of transmitting half tones by telefacsimile, thevarious shade values are converted into a frequency-modulated carrierwhich is required to handle a range of frequencies, for example be tween1800 C. P. S. representing black shades and 3000 C. P. S. representingwhite shades, the intervening shades being represented by interveningportions of the 1800-3000 cycle spectrum. Such frequency converters canbe used to transmit either class of subject matter. However, theelectrical elements of the converter introduce the equivalent of a timelag, for example in scanning sudden changes from black to white and viceversa.

A principal object of this invention is to provide an improved signalpeak-limiting arrangement.

A feature of the invention relates to a keying a control arrangement fortransmitting marking and spacing signals, wherein one signal is raisedto a very much higher level than the other and 4 Claims. (01. 178-44)Fig. 1 is a schematic wiring diagram of a facof the invention.

both are passed through a special limiting device so that in the outputonly one signal is present even though both signals are present at theinput.

A further feature relates to a keying arrangement employing two sourcesof oscillations, one representing marking condition and the otherrepresenting spacing condition, or vice versa and with the relativelevels of the two signals greatly different; in conjunction with a powerlimiting arrangement whereby switching from spacing to marking and viceversa is controlled by the swamping action of one signal on the other inpassing through the limiter.

A further feature relates to a novel form of threshold andpower-limiting arrangement.

A still further feature relates to the novel organization, arrangementand relative interconnection of parts whereby a simple and efficienttransmission of black and white facsimile subjects can be effected.

Other features and advantages not specifically enumerated will beapparent after a consideration of the following detailed descriptionsand the appended claims.

In the drawing which shows one preferred form,

Figs. 2, 3 and 4 represent modifications of a portion of the system ofFig. 1.

Referring to the drawing, numeral l represents any well-known form ofsignal generator such for example as a facsimile transmitting machine ofknown type having means to scan the successive elemental areas of thesubject matter to be transmitted whereby the shade values of such areasare translated into a corresponding electrical signal, e. g., an audiofrequency carrier signal. For a detailed description of such a device,referen'ce may be had to Patent No. 2,209,719. The generator I isconnected through an adjustable resistancefpad 2 to a couplingtransformer 3. In the case of black and white subject matter, devices Iand 2 are adjusted so that the facsimile signal is of some predeterminedfixed maximum level when a black area is being scanned, and thisleveldrops substantially near to zero when a white areais being scanned. Inother words, for black areas there appears at the primary winding oftransformer 3 an audio frequency signal, e. g., 1800 C. R8. of highlevel, e. g, 30 decibels, whereas for white areas there is substantiallyno signalapplied to transformer 2, or if there is, itis of the lowestpossible amplitude. f

' In theevehtthesignal from device I corresponding to white areas is notat the desired minimum amplitude, for example because of leakageconditions in the facsimile equipment, the signalsjare passed through athreshold limiter. This limiter comprises a pair of diodes 4, 5, whichmay be in separate tubes or they may be mounted within the same tube butelectrically isolated from each other. The signal from transformer 2 isapplied to cathode 4a directly, and to anode Eb through condenser G'andresistor 1. For negative half cycles the space between cathode 4a andanode 4b is conductive thus applying the signal to grid 13a of thetriode I3. On the positive half cycles, diode 5 becomes conductivebetween cathode 5a and anode 5b applying the signal through thecondenserfl and resistor 9. Resistors II! and ll are merely grid return,resistors for a tube is. The anode I31) is supplied with steady positivepotential from a suitable power supply, the positive terminal of whichis indicated in the drawing. A potentiometer l2 and voltage dividerresistor I4 are connected across the power supply source, and anotherset of voltage divider resisters l5, it, are connected across the powersupply for purposes to be described.

When a positive bias is applied from potentiometer l2 through thesecondary of transformer 3 to'cathode 4a and through resistor 9 tocathode 5a, each diode is non-conductive until 3 the applied signal fromtransformer 3 is greater than this bias. This condition obtains duringthe negative half cycles on diode 4 and for the positive half cycles ondiode 5. Thus the diodes act as threshold limiters for both the negativeand positive half cycles of the signal waves. Instead of providing thecathodes 4a and 5a with a positive bias and tying the respective anodesto ground, the cathodes can be tied to ground and a negative biasapplied to anodes 4b, 51); or in the alternative the cathode of onediode may be positively biassed while simultaneously a negative bias isapplied to the anode of the other diode. These two alternativearrangements are schematically illustrated in Figs. 2 and 3.

The tube I3 is coupled through a transformer H to a pair of balanceddiodes l8a, I8b, for fullwave rectification, the cathodes l9 and 20 ofthe two diodes being connected in balanced relation across the secondaryof transformer whose electrical midpoint is grounded.

Since maximum output is desirable from tube l3, transformer should be astep-up transformer and tube l3 should be biassed by grid bias resistor33 sothat tube l3 operates as a class A amplifier or as a class ABamplifier. Tubes H311 and |8b feed a tube 2| which is of thedouble-triode type, the triodes being arranged to cut off plate currentto their respective anodes for relatively low grid-bias voltages ontheir respective grids 2|a, 2|b. The plates of tube 2| are connected inbalanced relation to the primary of transformer 22 and the plate voltageis applied through voltage divider l5, l6, so that tubes 2| operate at amuch reduced plate voltage. The common bias resistor 23 is adjusted soas to provide a bias for grids 2 la, 2|b, approximately half way betweenzero bias and plate-current cutoff bias. Resistor 24 is preferably ofthe same value as resistor 23 and plates 25 and 26 of the diodes areconnected in balanced relation through resisters 21 and 28. The positivebias on the anodes 25 and 26 of the two diodes will equal the negativebias on the grids 2 la and 2|b thus providing a balanced and symmetricalarrangement. The tubes 18a, I81) and 2| comprise the peak limiter. Whenthe grids of tube 2| swing negatively beyond plate current cut-off,complete and effective limiting results. However, if the grids swingpositively into the grid current region, only partial limiting resultsand high distortion occurs. Tube 2| is biassed by resistor 23 half waybetween zero grid bias anda negative grid bias sufficient to cut off theplate current of tube 2|, then tube 2| will be operating in asubstantially linear portion of its characteristic. On signal swingsvwhich would tend to drive the grids into the conducting region, limitingis produced by the diodes |8a and Hit). Since the anodes of diodes |8aand |8b are biassed positively by resistor 2.4. which is equal in valueto resistor 23, the anode to cathode voltage of the diodes is equal tothe cathode to grid voltage of each portion of tube 2|. Thus when thesignal appears across the secondary of transformer assume that the upperhalf swings ne atively and the lowerhalf positively. Diode |8a willconduct current on the furthest excursion of its negative swing andthis'negative voltage is applied to grid -2|-a. When this voltage issuflicient to cut off the upper half of tube 2|, peak limiting occurs.At the same time the cathode of diode |8b was swinging: positive. Thisvoltage will appear on the anode of diode |8b only up to the pointwherethe cathode voltage is brought. up equal to the fixed bias on theanode 26, due to resistor 24. As the positive voltage on cathode 20becomes still further positive, diode |8b will cease to conduct and peaklimiting on the positive swing occurs. Since the voltage across resistor23 and resistor 24 are equal, peak limiting on the negative half cycleon the upper half of the cir- Therefore, on positive half cycles thediodes lfia.

IBb, will be respectively conductive up to the point of grid currentflow in tube 2! thus limiting the signal waves in the positivedirection. On negative half cycles the diodes Illa, 581), areconductive, but the grids of tube 2| are driven beyond plate currentcutoff, thus limiting the signal waves in the negative direction.Consequently, the peak limiter is symmetrical to both halves of thesignal wave cycles, as well as being balanced because of the push-pullarrangement. This symmetry has the advantage that only odd harmonics aregenerated and filter 29 may be made to attenuate only the third andhigher harmonics so that even the poorest filter will have little efiecton the fundamental frequencies. The signals from the filter 29 aretherefore substantially sinusoidal and can be transmitted over anychannel such as a radio channel to the receiving apparatus 30. If theapparatus 30 is a facsimile receiving machine, it will be operated insynchronism with the machine l as is well understood in the art and willbe provided with a recording mechanism which is selectively responsiveto the marking and spacing signals so as to reproduce the Whites andblacks of the original subject matter in the known manner.

Connected through a resistance 3| to a suitable point between resistorsI0 and I is a source 32 of oscillations of substantially uniformamplitude but of a different frequency from the black facsimil signalfrequency. For example, source 32 may produce a 3000 C. P. S. signal forpurposes to be described. Merely for explanatory purposes the 1800 C. P,S. signal from device will be called the marking signal, while the 3000C. P. S. signal from source 32 will be called the spacing signal. Thelevel of source 32 is adjusted to the point of limiting by the peaklimiter, but not beyond. I have found that for complete squelching orblanking of the signal from source 32, the level of the signal frommachine I must be about 20 decibels greater than the level of the signalfrom source 32 or in a voltage ratio of 10 to 1.

Since the local oscillator 32 is permanently connected in circuit, whenthe machine is in operation, two signals are applied to the peaklimiter, namely an 1800 C. P. S. marking signal from machine Irepresenting black elemental areas of the subject matter being scanned,and a 3000 C. P. S. spacing signal from oscillator 32. I have found thatwhen the power level of the marking signal is many times higher than thepower level of the spacing signal, the presence of the marking signalcompletely masks the spacing signal in the output of the limiter. Forthe best results, the marking signal should be about 20 decibels or morehigher than the spacing signal. Thus the marking signal may be of theorder of 30 decibels while the spacing signal may be decibels or less.

The operation of the system is as follows. The spacing signal fromsource 32 is adjusted to such a level that it is just barely limited onits peaks by the limiter. In other words, it just reaches the saturationpoint of the limiter so that any drop in the spacing signal will give adrop in the output from tube 2! but any increase of the spacing signalwill ive no further increase in the said output. Consequently, when nomarking signal is being received from the device I, a slightly flattenedsignal wave appears across transformer 22, at the spacing frequency butof a fixed maximum amplitude. When the device I is to be set inoperation, it is adjusted so that its power level at the input to thelimiter is many times the level of the spacing signal, preferably of theorder of 20 decibels higher. When a marking signal is applied to tube I3from the facsimile device I, it is preferably at least 10 times thevoltage level of the signal applied at the grid of tube I3 from source32. Under these circumstances, at the output of the peak limiter,namely, at transformer 22, there will appear substantially only themarking signal and it will be of the same amplitude as the spacingsignal would be in the absence of the marking signal. With a level ratioof 10 times between the marking and spacing signals, there remainssubstantially no vestige of the spacing signal in the output of thelimiter,

Because of the characteristics of the particular limiter described,there will be substantially no time delay or transients in the keying orswitching between the marking and spacing signals, nor will there be anychance of interaction between the two signals since one is always manytimes the other. The spacing and marking signals are transmited over thechannel L to a suitable facsimile receiver 30 which may be of the typedescribed in Patent No. 2,299,937, the electrical portions of which maybe as described in said patent and the mechanical portion of which maybe as described in Patent No. 2,209,719. As a result, the 1800 C. P. S.signal is translated into a black area on the receiving paper or filmwhile the 3000 C. P. S. signal is translated into a White area on thispaper or film, it being understood that the facsimile transmittingmachine and the facsimile reproducing machine are operated insynchronism as well-known in the art.

While in the foregoing, the system has been described as employing athreshold limiter between the facsimile machine I and the peak limiterI8aI8b-2I, in certain instances, the threshold limiter may not benecessary. Thus, regardless of the actual signal level corresponding towhite and black as delivered by the machine I, if the difference betweenthe two signals is relatively great, e. g., 40 decibels or better, thelevel of the signals from the facsimile machine with respect to thelevel of the signals from source 32 can be adjusted so that a markingsignal from machine I will squelch or blank the signal from source 32while on a spacing signal from machine I the signal from source 32 takescontrol. For example, if the marking signal from the facsimile machineis adjusted to 40 decibels greater than that required to reachsaturation in the peak limiter, and if the source 32 is adjusted to 20decibels greater than the said saturation level, then the marking signalfrom the facsimile machine being 20 decibels greater than the signalsfrom source 32, the latter would be blanked.

However, on a space signal from the facsimile machine, the source 32would be 20 decibels greater and would then squelch the space signalfrom the facsimile machine. This modification however is not asdesirable as that illustrated in Fig. 1 since it requires a peak limiterof much greater range'and the adjustments for the relative blankingconditions are more critical.

Various changes and modifications may be made in the disclosedembodiment without departing from the spirit and scope of the invention.While in the foregoing the tubes I3 and 2| are shown as of the triodetype, it will be understood that multigrid tubes may be employed.Furthermore, while tube 2| is shown as a single tube containing two setsof triodes, it will be understood that two separate triodes can beemployed. Consequently, in the claims the expression grid-controlledtubes includes arrangements where the sets of electrodes, e. g.,triodes, are in a single envelope or in separate envelopes. Fig. 4 showsa modification of Figs. 2 and 3 wherein the cathode 4a of tube 4 and theanode 5b of tube 5 are connected directly back-to-back, with the cathode5a positively biassed with respect to its anode 5b, and with the anode4b negatively biassed with respect to its cathode 4a.

This application is a division of application Serial No. 424,946, filedDecember 30, 1941, now issued as Patent No. 2,356,361, August 22, 1944.

What I claim is:

1. A signal peak limiter comprising a pair of diodes, a pair ofgrid-controlled amplifiers, a signal input circuit, means connectingsaid diodes to said input circuit for full wave rectification, a pair ofload resistors connected in series across the plates of said diodes, adiode biasing resistor connected from the junction point of said loadresistors to the midpoint of said input circuit, another bias resistorfor said amplifiers connected between said junction point and thecathodes of said amplifiers, said bias resistors being proportioned withrespect to each other so as to bias the anodes of the diodes and thegrids of the amplifiers substantially equally but opposite in sign sothat peak limitation will occur on substantially the same voltage forpositive half waves applied to one diode as for negative half waves aplied to the other diode.

2. A signal peak limiter according to claim 1 in which the control gridsof said amplifiers are normally biased between zero bias andapproximately one half plate current cutoif bias.

3. A signal peak limiter according to claim 1 in which both of said biasresistors are approximately equal.

4. A signal peak limiter according to claim 1 in which all of saidresistors are proportioned so that the diodes become conductive ofpositive input signal cycles only up to the point where the amplifiersbegin to draw grid current.

ROBERT M. SPRAGUE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,277,000 Bingley Mar. 17, 19422,171,671 Percival Sept. 5, 1939 2,298,657 Smith et a1 Oct. 13, 1942

